PAYPAL Chargeback - I've proof - what next?

I am from India and I worked with a US guy 4 months ago on a JV and he had sent some money via paypal in phases. I have all the proof of the money spent for business and he acknowledged that.. He then decided not to invest further citing personal reasons and asked me to shut down the JV and sell off all assets and send the money. Before sending money, I asked him to sign an agreement that clearly stated there are no dues from my end to be paid and all payments he made via paypal were authorized and fulfilled. He signed it and sent me the document after which I sent him the payment I received from selling the items off. He then cleverly opened a chargeback on paypal saying he didn't authorize payment and paypal has put my account balance in negative.

I uploaded proof and the agreement we had and paypal does nothing about it and he is now on a chargeback spree (chargingback all payments he made)..I am contemplating of taking legal action against him because he is clearly going against the rules of business. Anyone to offer suggestions on what I must do from here on?

One thing's for sure. I am not going to pay a dime to Paypal or to that fraud.. I called up Paypal and reported this issue and the CSR told they cannot stop a buyer from claiming chargebacks with his/her credit card company, but how the hell can credit card company refund to him without verifying if he is simply scamming someone out of it?